(a) Field of the Invention
The present invention relates to an ink jet printing head and a method for driving the same. More specifically, it relates to an ink jet printing head for an ink jet printer having a piezoelectric element for converting an electric signal to mechanical energy levels and to a method for driving the ink jet printing head.
(b) Description of the Related Art
A non-impact printing system has attracted special interest lately because of its small noise and high-speed printing. Among other non-impact printing systems, an ink jet printing system, in which liquid ink droplets are discharged from a printing head and adheres to recording paper to thereby form characters or figures, has an advantage that high-speed printing is performed on plain paper without a special fixing process. Various types of ink jet printers using the ink jet printing system have been proposed and manufactured.
The ink jet printing systems are roughly categorized in three types including a continuous injection type, an impulse injection type (or on-demand type) and an electrostatic attraction type. Especially, the on-demand type is expected for practical use because of its advantages of a low ink consumption and a simple structure. The advantages may be attributed to the piezoelectric elements operated to discharge liquid ink droplets on each demand. Examples of the publications disclosing on-demand type ink jet printers or methods for driving the same includes Patent Publication Nos. 59(1984)-98862, 63(1988)-251241, 1(1989)-101160, 1(1989)-297258 and 3(1991)-213346.
Patent Publication No. 59-98862 describes a method for driving an ink jet printing head in which a plurality of driving pulses are supplied to piezoelectric elements synchronously with the natural period for oscillation of the ink head which is smaller than the minimum response period of the piezoelectric elements, to thereby change the number of ink molecules per ink droplet in accordance with requested gray scale levels, while maintaining the printing speed.
Patent Publication No. 63-251241 describes an ink jet printing head in which an ink droplet is first discharged from an orifice by rapidly reducing the volume of the ink chamber for pressurizing, then the volume of the ink chamber is increased slowly so that movement of the meniscus in the ink nozzle after discharge of the ink droplet is restricted within a predetermined amount and a limited speed. To attain this movement of the meniscus, the ink printing head has a signal modulating section for changing the time constant in a fall time of the driving pulses in accordance with the voltage level of the driving pulses applied to the piezoelectric elements, thereby controlling the time period for recovering the ink chamber to the initial state. It is described that this type of ink head printer has advantages of superior frequency response, stable discharge, fine gradation levels and imaging accuracy.
Patent Publication No. 1-101160 describes an on-demand type ink jet printer in which a supplementary pulse is applied to piezoelectric elements after a printing pulse is supplied to the piezoelectric elements, the supplementary pulse having a delay time in accordance with the gray scale levels to thereby operate the printing head in a gradation sequence in accordance with the information supplied thereto.
Patent Publication No. 1-297258 describes a method for driving an ink jet printing head in which the electric signal supplied to the piezoelectric elements includes a first pulse for discharging ink droplets from the ink nozzle and a second pulse having a waveform substantially equal to the waveform of the first pulse and a delay time of 2l/c from the first pulse, wherein l is the length of the portion of the printing head which corresponds to the length of the piezoelectric elements and reflects pressure wave, and wherein c is the sound velocity along the ink inside the ink chamber. The method also changes the fall time of the driving pulses dependently of the ink temperature detected by a thermal sensor. The method has an advantage that satellite ink droplets are reduced. In general, the satellite droplet degrades the imaging quality due to the difference in landing position of the ink, which is caused by the difference in velocity between the satellite droplets and the main droplets.
Patent Publication No. 3-213346 describes an ink jet printer having a changing means for changing the amount of discharged ink dependently of the driving timing of the piezoelectric element and a delay means for delaying the driving timing of the piezoelectric elements from the timing of the operation of the changing means, wherein the mount of the discharged ink is changed dependently of the delay time by the delay means. The ink jet printer has an advantage in obtaining a uniform printing in a gradation sequence or gray scale level printing.
The ink jet printing system is expected to attain a full-color image by changing the size or diameter of the discharged ink droplets in a gradation sequence printing. Examples of such printing systems so far proposed include one having means for changing the driving voltage applied to the piezoelectric elements, one having different ink chambers for receiving inks having different concentrations, one having a plurality of ink nozzles having different diameters for a gradation sequence printing, etc.
Those proposed ink Jet printing systems as described above, however, do not always provide a sufficient frequency response, stable discharge of the ink droplets, excellent image in gradation sequence, and accurate imaging positions. For example, the method controlling the driving voltage of the piezoelectric elements for a gradation sequence printing has a problem that the velocity of the ink droplets changes depend on the voltage levels so that landing positions of the ink droplets change accordingly, resulting in degradation in the imaging quality.
Further, the rise in the voltage level in the driving voltage generates satellite droplets which have low velocities and degrade the printing quality. The fourth publication as mentioned above describes the technique for reducing the satellite droplets. However, it is difficult to entirely remove the satellite droplets even by this technique.
The ink jet printer in which different ink concentrations provide different gray scale levels or in which different nozzles having different diameters provide different gray scale levels also have the disadvantages that the printer has a large dimension and requires a large cost for production.